Finely controlled size-selective precipitation and separation of CdSe/ZnS semiconductor nanocrystals using CO2-gas-expanded liquids

Process Principles for Large-Scale Nanomanufacturing

Nanomanufacturing—the fabrication of macroscopic products from well-defined nanoscale building blocks—in a truly scalable and versatile manner is still far from our current reality. Here, we describe the barriers to large-scale nanomanufacturing and identify routes to overcome them. We argue for nanomanufacturing systems consisting of an iterative sequence of synthesis/assembly and separation/sorting unit operations, analogous to those used in chemicals manufacturing. In addition to performance and economic considerations, phenomena unique to the nanoscale must guide the design of each unit operation and the overall process flow. We identify and discuss four key nanomanufacturing process design needs: (a) appropriately selected process break points, (b) synthesis techniques appropriate for large-scale manufacturing, (c) new structure- and property-based separations, and (d) advances in stabilization and packaging.

Enhanced cellular uptake of size-separated lipophilic silicon nanoparticles

Specific size, shape and surface chemistry influence the biological activity of nanoparticles. In the case of lipophilic nanoparticles, which are widely used in consumer products, there is evidence that particle size and formulation influences skin permeability and that lipophilic particles smaller than 6 nm can embed in lipid bilayers. Since most nanoparticle synthetic procedures result in mixtures of different particles, post-synthetic purification promises to provide insights into nanostructure-function relationships. Here we used size-selective precipitation to separate lipophilic allyl-benzyl-capped silicon nanoparticles into monodisperse fractions within the range of 1 nm to 5 nm. We measured liposomal encapsulation and cellular uptake of the monodisperse particles and found them to have generally low cytotoxicities in Hela cells. However, specific fractions showed reproducibly higher cytotoxicity than other fractions as well as the unseparated ensemble. Measurements indicate that the cytotoxicity mechanism involves oxidative stress and the differential cytotoxicity is due to enhanced cellular uptake by specific fractions. The results indicate that specific particles, with enhanced suitability for incorporation into lipophilic regions of liposomes and subsequent in vitro delivery to cells, are enriched in certain fractions.

Stable monodisperse nanomagnetic colloidal suspensions: An overview

Magnetic iron oxide nanoparticles (MNPs) have emerged as highly desirable nanomaterials in the context of many research works, due to their extensive industrial applications. However, they are prone to agglomerate on account of the anisotropic dipolar attraction, and therefore misled the particular properties related to single-domain magnetic nanostructures. The surface modification of MNPs is quite challenging for many applications, as it involves surfactant-coating for steric stability, or surface modifications that results in repulsive electrostatic force. Hereby, we focus on the dispersion of MNPs and colloidal stability.

Quantum dots: heralding a brighter future for clinical diagnostics

Quantum dots (QDs) are semiconductor nanocrystals that possess unique optical properties including broad-range excitation, size-tunable narrow emission spectra and high photostability, giving them considerable value in various biomedical applications. The size and composition of QDs can be varied to obtain the desired emission properties and make them amenable to simultaneous detection of multiple targets. Furthermore, numerous surface functionalizations can be used to adapt QDs to the needed application. The successful use of QDs has been reported in the areas of in vitro diagnostics and imaging. There is also potential for multimodal applications for simultaneous imaging. Toxicity issues are still a prime concern with regards to in vivo applications on account of the toxic constituents of QDs.

Small silicon, big opportunities: the development and future of colloidally-stable monodisperse silicon nanocrystals

Nanomaterials are becoming increasingly widespread in consumer technologies, but there is global concern about the toxicity of nanomaterials to humans and the environment as they move rapidly from the research laboratory to the market place. With this in mind, it makes sense to intensify the nanochemistry community's global research effort on the synthesis and study of nanoparticles that are purportedly "green". One potentially green nanoparticle that seems to be a most promising candidate in this context is silicon, whose appealing optical, optoelectronic, photonic, and biomedical attributes are recently gaining much attention. In this paper, we outline some of our recent contributions to the development of the growing field of silicon nanocrystals (ncSi) in order to stress the importance of continued study of ncSi as a green alternative to the archetypal semiconductor nanocrystals like CdSe, InAs, and PbS. While a variety of developments in synthetic methods, characterization techniques, and applications have been reported in recent years, the ability to prepare colloidally-stable monodisperse ncSi samples may prove to have the largest impact on the field, as it opens the door to study and access the tunable size-dependent properties of ncSi. Here, we summarize our recent contributions in size-separation methods to achieve monodisperse samples, the characterization of size-dependant property trends, the development of ncSi applications, and their potential impact on the promising future of ncSi.

Size-dependent absolute quantum yields for size-separated colloidally-stable silicon nanocrystals

Size-selective precipitation was used to successfully separate colloidally stable allylbenzene-capped silicon nanocrystals into several visible emitting monodisperse fractions traversing the quantum size effect range of 1-5 nm. This enabled the measurement of the absolute quantum yield and lifetime of photoluminescence of allylbenzene-capped silicon nanocrystals as a function of size. The absolute quantum yield and lifetime are found to monotonically decrease with decreasing nanocrystal size, which implies that nonradiative vibrational and surface defect effects overwhelm spatial confinement effects that favor radiative relaxation. Visible emission absolute quantum yields as high as 43% speak well for the development of "green" silicon nanocrystal color-tunable light emitting diodes that can potentially match the performance of their toxic heavy metal chalcogenide counterparts.

Cylindrical-to-spherical shape transformation of lecithin reverse micelles induced by CO2

The effect of CO(2) on the microstructure of L-alpha-phosphatidylcholine (lecithin) reverse micelles was studied. The small-angle X-ray scattering (SAXS) results show that CO(2) could induce a cylindrical-to-spherical micellar shape transformation. Fourier transform infrared (FT-IR) and UV-vis techniques were also utilized to investigate intermolecular interactions and micropolarity in the reverse micelles at different CO(2) pressures. The reduction of the degree of hydrogen bonding between surfactant headgroups and water with added CO(2) was found to be the main reason for the micellar shape transformation. In the absence of CO(2), the hydrogen bonding between water and P=O of lecithin forms a linking bridge in the interfacial layer. Therefore, the free movement of the polar head of lecithin is limited and the cylindrical reverse micelles are formed. Upon adding CO(2) to the reverse micelles, the hydrogen bonds between lecithin and water in reverse micelles are destroyed, which is favorable to forming spherical micelles. Moreover, the CO(2)-combined reverse micelles were utilized in the synthesis of silica particles. Rodlike silica nanoparticles were obtained in the absence of CO(2), and ellipsoidal and spherical mesoporous silica particles were formed in the presence of CO(2). This method of tuning micellar shape has many advantages compared to traditional methods.

Size-selective fractionation of nanoparticles at an application scale using CO2 gas-expanded liquids

Size-based fractionation of nanoparticles remains a non-trivial task for the preparation of well-defined nanomaterials for certain applications and fundamental studies. Typical fractionation techniques prove to be inefficient for large nanoparticle quantities due to several factors including the expense of equipment, throughput constraints, and the amount of organic solvent waste produced. Through the use of the pressure-tunable physico-chemical properties of CO2-expanded liquids, a rapid, precise, and environmentally sustainable size-selective fractionation of ligand-stabilized nanoparticles is possible through simple variations in applied CO2 pressure. An apparatus capable of fractionating large quantities of nanoparticles into distinct fractions with the ability to control mean diameters and size distributions has been developed. This apparatus consists of three vertically mounted pressure vessels connected in series with needle valves. This process, at current design scales, operated at room temperature, and CO2 pressures between 0 and 50 bar, results in a batch size-selective fractionation of a concentrated nanoparticle dispersion. This paper presents this new apparatus and the separation results of various single pass fractionations as well as recursive fractionations.

Enhanced stabilization of vesicles by compressed CO2

In this work, we studied the effect of compressed CO2 on the stability of vesicles formed in a dodecyltrimethylammonium bromide (DTAB)/sodium dodecyl sulfate (SDS) mixed surfactant system by combination of phase behavior and turbidity study, and UV-vis and fluorescence techniques. It was discovered that compressed CO2 could enhance the stability of vesicles significantly. This new and effective method to stabilize vesicles has some unique advantages over conventional methods. For example, the size and stability of the vesicles can be easily controlled by CO2 pressure; the method is greener because CO2 is a green reagent and it can be released completely after depressurization, which simplifies postseparation processes in applications. The main reason for CO2 to stabilize the vesicles is that CO2 molecules can insert into the hydrophobic bilayer region to enhance the rigidity of the vesicle film and reduce the size of the vesicles, which is different from that of conventional cosolvents (e.g., alcohols) used to stabilize vesicles. On the basis of this discovery, we developed a method to prepare hollow silica spheres using tetraethoxysilane as the precursor and CO2-stabilized vesicles as the template, in which CO2 acts as both the stabilizer of the vesicular template and the catalyst for the hydrolysis reaction of the precursor, and other cosolvents and catalysts are not required. Besides, the size of the silica hollow spheres prepared can be controlled by the pressure of CO2.

Gold nanoparticle deposition using CO2 expanded liquids: effect of pressure oscillation and surface-particle interactions

Postsynthesis processing of nanoparticles to obtain mesoscale hierarchal nanostructures is the key for the development of nanotechnology and smart composites/coatings from these materials. We have utilized gas-expanded liquid deposition of alkyl-coated gold nanoparticles to study the effects of variable process flowrates, variable flow oscillation and variable interaction potential of the substrate on nanoparticle array quality. Array quality is measured here as completeness of area surface coverage of approximately a monolayer of nanoparticles. Quantitative values for surface coverage are averages obtained from multiple TEM photomicrographs using Image J digital analysis. The process was modified using higher CO2 addition rate outside of the pressure range necessary for deposition, and this modified process produced an excellent film quality while reducing overall processing time by 45%. The effects of pressure oscillation during deposition appeared to anneal the film at the lower flow rates, 0.5 and 1.0 mL/min, but a reduction in area coverage was observed with pressure oscillation at 3.0 mL/min. Pressure oscillation has emerged as a useful tool for researchers to tune the film uniformity and therefore the surface roughness. Calculations based on Hamaker theories for surface-particle interactions on various substrates were performed, and better surface coverage was predicted for C-based surfaces compared to Si3N4 and SiO2 surfaces. Indeed, experimental studies verified this general ordering, indicating that if surface interactions with the particles are strong deposition directly on the surface rather than on pre-existing nanoparticle islands may govern uniform deposition.

Biocompatible, lactide-based surfactants for the CO2-water interface: high-pressure contact angle goniometry, tensiometry, and emulsion formation

The unique properties of compressed CO2, including its low cost, nontoxicity, easily tunable solvent strength, and favorable transport properties, make it an environmentally attractive alternative to volatile organic solvents. Suitable surface-active species can be utilized to realize the full potential of clean, CO2-based technologies, by helping to overcome the low solubility typically associated with many solutes of interest in CO2. In this work we synthesize and investigate the interfacial activity of a series of nonionic amphiphiles with a biocompatible and biodegradable CO2-phile at both the CO2-water (C|W) and CO2-water-solid (C|W|S) interfaces. We developed a high-pressure pendant drop tensiometer and contact angle goniometer that allows us to measure both tension and contact angle in tandem. The tension of the C|W interface was measured in the presence of the lactide (LA)-based surface active agents with varying molecular weight and hydrophilic-to-CO2-philic ratios. Emulsion studies with an optimum balanced surfactant were performed. The contact angle of water droplets against a silane-modified (hydrophobic) substrate under CO2 atmosphere was also measured in presence of a selected LA-based amphiphile. The results demonstrate that the nonionic copolymers with the biodegradable and biocompatible LA-based group can significantly reduce the tension of the C|W interface. The LA-based surface active species are also capable of forming stable emulsions of water and CO2 and reducing the angle of the three-phase C|W|S contact line.